1. Field of the Invention
The invention relates to lasers and specifically to a high power laser structure using a unique cooling structure.
2. Description of the Related Art
Conventional ion lasers include a resonant cavity defined by a series of two reflecting optical elements placed at either end of a laser tube. One of the elements usually comprises an output coupler. In general, the optical elements are externally connected to the laser tube; that is, the mirrors are mounted directly to the ends of the tube, with a glass frit, or solder glass joining the reflective portion of the element directly to the end of the laser tube.
Another laser design incorporates optics not mounted directly to the tube, but mounted to a separate external support structure, which supports both the tube and the optics. Such a prior art design is shown in FIG. 1, and includes laser tube 11, including a bore 12, manufactured from a ceramic material such as beryllia (BeO) and a cooling assembly 17, mounted in a support structure 14. A magnet assembly 21 may also be provided to help concentrate laser discharge at the center of bore 11. Mirrors 15,16 are mounted and supported at either end of laser tube 11 in support structure 10. While structures such as support structure 14 allow for relatively easy adjustment of optics 15 and 16, they are relatively complicated to manufacture, requiring great precision, and can be problematic over time due to mechanical deterioration.
U.S. Pat. No. 4,893,314, inventors Shull, et al., assigned to Spectra-Physics, Inc., shows an optics mounting alternative wherein the laser optics are mounted to a mounting assembly which is itself coupled in the ends of the laser bore. A mirror seat is inserted into the interior of the laser bore at each respective end and the optical element is mounted on the mirror seat with the reflective portion of the element on the opposite side of the mounting area. As discussed in detail in co-pending application Ser. Nos. 07/950,415 and 07/987,960, this particular mounting structure presents a number of advantages over the external mirror mount structure shown in FIG. 1.
The optical elements for a laser resonator may be manufactured by coating a substrate, such as glass, with a series of dielectric films to develop the desired reflectance/transmittance of the mirror, depending on whether the mirror is to be used as a high reflectance mirror or as an output coupler. As is well known, such coatings are generally comprised of a plurality of layers of dielectric material, the layers in a reflective stack alternating between materials with high and low indices of refraction with each layer being typically about .lambda./4 in optical thickness, thereby defining a reflective surface.
In ion lasers where a "white light" output is desired, laser operation over a broad range of wavelengths--the red, green and blue visible regions of the spectrum--is required, thus necessitating that the optical elements reflect and transmit over this broad range. High power white light lasers (having on the order of one watt output power) are useful in applications such as artistic laser light displays, wherein the beam will be split into its spectral components to make multi-colored designs.
In general, to generate sufficient output power, input power several hundred times the desired output power is required to produce the desired output. For example, it is generally necessary to provide as much as 3-8 kilowatts of input power to generate 1-2 watts of visible laser light. In such high energy lasers, much of this energy is dissipated in the form of heat and thus cooling the laser becomes a significant design concern. Generally, circulating air over the laser bore provides sufficient cooling only in low power applications. For high power lasers, cooling using a recirculating coolant is generally required.
As shown in FIG. 1, water cooling generally involves surrounding the exterior portion of the laser tube with a sealed sheath to allow water to flow along the exterior of the ceramic bore in direct contact therewith. Water generally circulates over the bore to remove heat therefrom and to transmit the heat to a heat exchanger. Because water flows directly over the bore, in contact therewith, it is preferable to have a tube manufactured from a solid piece of ceramic material. Tube 12, shown in FIG. 1, is of the type conventionally used with a noble gas ion laser. Tube 12 is generally filled with a gas such as argon, krypton, or a combination thereof, and has a first end 13a, where an anode A is generally located and a second, larger end 13b, where a cathode assembly C is located. Second end 13b also serves as a gas reservoir.
Cooling assembly 17 includes a coolant inlet 17a and coolant outlet 17b, which surrounds both the narrow portion of laser bore 12 and the cathode portion 13b. O-rings 19 are provided at each end of the bore to seal the sheath 18. When using water cooling with bore 12, safety and reliability problems may develop if there is interaction between the water and the electrical couplings due to deterioration of the seals 19 adjacent anode A and cathode C. The materials used to seal the laser tube tend to deteriorate over time allowing water coolant to interact with the anode or cathode, resulting in electrolysis and even loss of vacuum integrity.
Further, while it is preferable to have a single piece ceramic bore 12 and cathode reservoir volume 13b, such tubes are extremely costly to manufacture.
In a one-piece ceramic tube, the ceramic material must be formed to a diameter sufficient to encompass the gas reservoir region 13b. The internal bores and exterior surface are then machined to achieve the shape depicted in FIG. 1. Two-piece tubes-with a ceramic, narrow bore 13a and a ceramic or metallic reservoir region 13b--have been developed to reduce the costs associated with manufacture a single piece bore. With a two-piece tube, a bore region and a reservoir region are joined at a junction 23. However, sealing this joint is problematic when developing reliable, high power, water cooled lasers.